The project seeks to provide information on the existence and anatomical definition and extent of steroid hormone target cells in the brain of phylogenetic representatives of different vertebrate classes and in neonatal and fetal rats, determining also the ontogenetic appearance of steroid "receptors". In depth studies with 3H estradiol-17 beta in the rat are aimed at the quantitative assessment of differences in estrogen-"receptor" concentration related to different postnatal developmental states, male and female sex, and organizational and activational hormone induced modifications. Characterization of estrogen target cells will be attempted by using recently developed combinations of catecholamine fluorescence or immunohistochemical staining with anti-gonadotropin with the autoradiographic 3H estradiol localization as well as by using electromicroscopic autoradiography. Apparent differences in "receptor" affinity to estradiol or antagonists within brain regions and between brain, pituitary and uterus will be measured and evaluated quantitatively. Pharmacological antagonists will be tested as to tissue selective suppression of estrogen "receptor" binding. Selective inhibition of "receptor" binding could be of therapeutic potential.